1. Field of the Invention
The present invention relates to a flat panel display device and more particularly, to an electroluminescent display (ELD) device and a method of fabricating an electroluminescent display (ELD) device.
2. Discussion of the Related Art
In general, organic electroluminescent display (OELD) devices have an electron supply electrode, which is commonly referred to as a cathode electrode, and hole supply electrode, which is commonly referred to as an anode electrode. Accordingly, electrons and the holes are input into a light-emitting layer from the cathode and anode electrodes, respectively, wherein the electrons and holes together form exciton pairs and light is emitted when an energy of the exciton is reduced from an excited state to a ground state. Accordingly, since OELD devices do not require additional light sources, both volume and weight of the OELD devices may be reduced, as compared to liquid crystal display (LCD) devices. In addition, the OELD devices are advantageous because of their low power consumption, high luminance, fast response time, and low weight. Currently, OELD devices are implemented in mobile telecommunication terminals, car navigation systems (CNSs), personal digital assistants (PDAs), camcorders, and palm computers. In addition, since manufacturing processes for OELD devices are relatively simple, manufacturing costs can be reduced, as compared to manufacturing costs of LCD devices.
The OELD devices may be classified into passive matrix-type OELD devices and active matrix-type OELD devices. Although the passive matrix-type OELD devices have simple structures and simplified manufacturing processes, they require large amounts of power to operate making them unsuitable for large-sized display devices, and their aperture ratios decrease as the number of electric lines increase. Conversely, the active matrix-type OELD devices have high light-emitting efficiency and high image display quality.
FIG. 1 is a cross sectional view of a passive matrix-type OELD device according to the related art. In FIG. 1, a plurality of pixel regions “P” are defined on a substrate 2, wherein a transparent first electrode 4 is formed on an entire surface of the substrate 2. In addition, a plurality of partitions 16 that have insulating properties are formed along boundary regions of neighboring pixel regions “P” on the substrate 2. A plurality of organic electroluminescent layers 18 corresponding to the pixel regions “P” are formed on the first electrode 4, and a plurality of second electrodes 20 are formed on each of the organic electroluminescent layers 18. Accordingly, the first electrode 4 functions as a hole supply electrode, which supplies holes to each of the organic electroluminescent layers 18, and the second electrodes 20 functions as electron supply electrodes, which supply electrons to each of the organic electroluminescent layers 18.
The partitions 16 protect the organic electroluminescent layers 18 from chemical deterioration during a development and etching processes for forming the second electrodes 20. Accordingly, the second electrodes 20 are formed separately within each of the pixel regions “P” using a shadow masking process. Thus, if the organic electroluminescent layers 18 are formed after formation of the partitions 16, the organic electroluminescent layers 18 are only formed on the first electrode 4 corresponding to the pixel regions “P” and on top surfaces of the partitions 16.
An insulating layer pattern 12, which has a larger area than the top surface of the partitions 16, is formed beneath the partitions 16 to prevent the second electrodes 20 from contacting the first electrode 4 during processes for forming the second electrodes 20. The organic electroluminescent layers 18 may have single-layered structures or multi-layered structures each having a light-emitting layer 18a, an electron carrying layer 18b, and a hole carrying layer 18c. 
FIG. 2 is a cross sectional view of an active matrix OELD device according to the related art. In FIG. 2, an OELD device 30 includes a transparent first substrate 32, a thin film transistor array part 34 having a first electrode 36, an organic light-emitting layer 38, and a second electrode 40, wherein the thin film transistor array part 34 is formed directly on the transparent first substrate 32, and the first electrode 36, organic light-emitting layer 38, and second electrode 40 are formed over the thin film transistor array part 34. The light-emitting layer 38 displays red (R), green (G), and blue (B) colors and is formed by patterning organic material(s) separately for each pixel for the R (red), G (green) and B (blue) colors. The organic electroluminescent display (ELD) device 30 is completed by bonding the first substrate 32 and a second substrate 48 together by disposing a sealant material 47 between the first and second substrates 32 and 48.
In FIG. 2, the second substrate 48 has a moisture absorbent desiccant 41 for removing moisture and oxygen that may infiltrate into an interior of the OELD device 30. The moisture absorbent desiccant 41 is formed by etching away a portion of the second substrate 48, filling the etched portion of the second substrate 48 with moisture absorbent desiccant material, and fixing the moisture absorbent desiccant material with tape 45.
FIG. 3 is a plan view of a thin film transistor array pixel part of an OELD device according to the related art. In FIG. 3, a pixel includes a switching element TS, a driving element TD, and a storage capacitor CST at every pixel region “P” defined on a substrate 32, wherein the switching element TS and the driving element TD are formed with combinations of more than two thin film transistors (TFTs), and the substrate 32 is formed of a transparent material, such as glass and plastic. A gate line 42 is formed along a first direction, and a data line 44 is formed along a second direction perpendicular to the first direction, wherein the data line 44 perpendicularly crosses the gate line 42 with an insulating layer between the gate and data lines 42 and 44 and a power line 55 is formed along the second direction, and is spaced apart from the data line 44.
The thin film transistor used for the switching TFT TS has a gate electrode 46, an active layer 50, a source electrode 56, and a drain electrode 60, and the thin film transistor for the driving TFT TD has a gate electrode 68, an active layer 62, a source electrode 66, and a drain electrode 63. The gate electrode 46 of the switching TFT TS is electrically connected to the gate line 42, and the source electrode 56 of the switching TFT TS is electrically connected to the data line 44. In addition, the drain electrode 60 of the switching TFT TS is electrically connected to the gate electrode 68 of the driving TFT TD through a contact hole 64, and the source electrode 66 of the driving TFT TD is electrically connected to the power line 55 through a contact hole 58. Furthermore, the drain electrode 63 of the driving TFT TD is electrically connected to a first electrode 36 within the pixel region “P,” wherein the power line 55 and a first capacitor electrode 35 that is formed of polycrystalline silicon layer form a storage capacitor CST.
FIG. 4 is a cross sectional view along IV—IV of FIG. 3 according to the related art. In FIG. 4, the OELD device includes the driving TFT TD, a first electrode 36, a light-emitting layer 38, and a second electrode 80, wherein the driving TET TD has a gate electrode 68, an active layer 62, a source electrode 66, and a drain electrode 63. Accordingly, the first electrode 36 is formed over the driving TFT TD and is connected to the drain electrode 63 of the driving TFT TD with an insulating layer 67 between the first electrode 36 and the driving TFT TD. The light-emitting layer 38 is formed on the first electrode 36 for emitting light of a particular color wavelength within an emission region E, and the second electrode 80 is formed on the light-emitting layer 38. A storage capacitor CST (in FIG. 3) is connected in parallel to the driving TFT TD, and includes first and second capacitor electrodes 35 and 55. The source electrode 66 of the driving TFT TD contacts the second capacitor electrode 55, i.e., a power line, and the first capacitor electrode 35 is formed of polycrystalline silicon material under the second capacitor electrode 55. The second electrode 80 is formed on the substrate 32 on which the driving TFT TD, the storage capacitor CST, and the organic light-emitting layer 38 are formed. Accordingly, the OELD device is a bottom emission-type OELD device, wherein the light-emitting layer emits the light downward through the substrate 32. In addition, each of the pixels having the driving TFT TD and the storage capacitor CST are separated by partitions formed between two adjacent pixels.
FIG. 5 is a cross sectional view along V—V of FIG. 3 according to the related art. In FIG. 5, a switching TFT Ts, a first electrode 76, an organic light-emitting layer 38, and a second electrode 80 are formed on the substrate 32. The switching TFT Ts has an active layer 50 formed of polycrystalline silicon, a gate electrode 46 on the active layer 50, a source electrode 56 and a drain electrode 60, wherein the first electrode 76, the organic light-emitting layer 38, and the second electrode 80 are sequentially formed over the switching TFT Ts. The organic light-emitting layer 38 is formed to have a multi-layered structure including a main light-emitting layer 38a, an electron-carrying layer 38b, and a hole-carrying layer 38c. Partitions 70 are formed within a boundary region of each pixel to insulate adjacent pixels and to maintain a necessary gap for supplying a shadow mask during processing. The partitions 70 have a trapezoidal shape, wherein a width gradually decreases from a bottom to a top of the partitions 70 to form the second electrode 80 on an entire surface of the substrate 32 and along the top surfaces of the partitions 70.
However, the passive-type OELD devices are not adequate for large-sized electroluminescent display devices. Conversely, although the active matrix-type OELD devices are suitable for large-sized electroluminescent display devices, the thin film transistor array part and the light-emitting part are formed on a same substrate. For example, active matrix-type OELD devices are commonly manufactured by forming the thin film transistor array part and the light-emitting part on a same substrate, and then bonding the substrate to an encapsulating structure. Accordingly, a panel yield having the thin film transistor array part and the light-emitting part is dependent upon the product of the individual yields of the thin film transistor array part and the light-emitting part. However, the panel yield is greatly affected by the yield of the organic light-emitting layer. Thus, if an inferior organic light-emitting layer that is usually formed of a thin film having a thickness of 1000 Å is defective due to impurities and/or contaminants, the panel is rejected as being an inferior quality panel, wherein production and material costs are lost and panel yield decreases.
Although bottom emission-type OELD devices have high image stability and variable fabrication processing, they are not adequate for implementation in devices that require high image resolution due to their increased aperture ratio. In addition, since top emission-type OELD devices emit light along an upward direction through the substrate, the light is not influenced by the thin film transistor array part that is positioned under the light-emitting layer. Accordingly, design of the thin film transistors may be simplified and the aperture ratio can be increased, thereby increasing the operational life span of the device. However, since a cathode is formed over the light-emitting layer in OELD devices, material selection and light transmittance are limited, whereby light transmission efficiency is reduced. In addition, if a thin film-type passivation layer is formed to prevent the reduction of the light transmittance, the thin film passivation layer may fail to prevent infiltration of ambient air into the device.